1. Field of the Invention
This invention relates to water soluble printed wiring board (PWB) spacers and more specifically to water soluble PWB spacers of thermoplastic materials which can be mass produced using one of the melt processable techniques applicable to thermoplastics such as injection molding.
2. Brief Description of the Prior Art
Mounts and spacers are often required to provide spacing for the electronic components at various elevations above PWBs. The purpose of these mounts and spacers is as follows:
(1) Provide adequate air space and lead length for heat dissipation to prevent operational overheating.
(2) Ability to clean under parts. Cleanability is an important design concern. Sufficient spacing between the part and the board will ensure that the cleaning solvent or aqueous cleaner can flow under the part and prevent entrapment of contaminants. As greater lead numbers and finer lead pitches are used, it becomes harder to force the cleaning solvents under the part.
(3) Provide mechanical protection. Eliminate lead stresses and improve shock and vibration characteristics.
(4) Simplify assembly, touch-up and repair work accessibility to leads for inspection.
The most common technique currently used to provide component spacing on the PWBs is with the use of mounts and spacers, produced from thermoplastics such as nylon. These mounts and spacers however become permanent to the printed wiring board assembly. The disadvantage include added cost of permanent mounts and spacers and
An optional technique used for elevating electronic components of the printed wiring board is by placing a slight twist on the leads of electronic components. This method, commonly referred to as jog form, is labor intensive and is applicable to less than 30% of applications. This method also promotes leaning over of components which in turn leads to printed wiring board rework and/or scrap.
The most pertinent prior art of which applicant herein is aware involves wash away spacers compounded of organic material (lactose) for spacing electronic components on printed wiring boards. The manufacturing technique of these spacers is unknown. Prior to flow/wave solder processing, the wash away spacers of various shapes and desired thickness are placed between electronic components and the printed wiring board to raise the electronic component height to a desired elevation. Most often the leads of the electronic components pass through the spacers inside diameter and finally through printed wiring board to keep spacer intact between the electronic component and the printed wiring board during flow solder process. The spacers can also be placed between electronic components and printed wiring board without leads passing therethrough to provide desired spacing between electronic components and the printed wiring board. These techniques can be expensive since lactose spacers are brittle, difficult to handle and can prematurely break before or during flow solder processing, creating massive rework and scrap of printed wiring boards.
In accordance with the present invention, the above noted problems of the prior art are eliminated and there is provided a relatively inexpensive to produce PWB spacer which is not brittle and subject to breakage, retains its shape at the PWB fabrication and soldering temperatures encountered, yet is water soluble and easily removed from the completed PWB.
Briefly, the desired water soluble thermoplastic PWB spacers are molded by using partially (approximately 88%) hydrolyzed polyvinyl alcohol resins with a melting range between 120xc2x0 C.-200xc2x0 C. and exhibit up to 24% crystallinity or by using fully (approximately 98%) hydrolyzed polyvinyl alcohol resins with a melting range between 150xc2x0 C.-230xc2x0 C. which exhibit up to 45% crystallinity. The partially hydrolyzed polyvinyl alcohol resins are cold water soluble, where as fully hydrolyzed polyvinyl alcohol resins are cold water insoluble.
Alternatively, a blend of the partially hydrolyzed polyvinyl alcohol resins and the fully hydrolyzed polyvinyl alcohol resins mentioned above can be used to alter the properties of the PWB spacers used in various applications. The ratio of the partially hydrolyzed polyvinyl alcohol resin to the fully hydrolyzed polyvinyl alcohol resin depends upon following factors:
1. The various temperatures involved during fabrication of the PWB to avoid premature melting of the spacers.
2. The degree of solubility in water required for the spacer.
A preferred source of partially hydrolyzed polyvinyl alcohol resin is 2000 series and 5000 series VINEX(trademark) thermoplastic polyvinyl alcohol copolymer resins manufactured by Air Products and Chemicals, Inc., Polymer Chemicals Division, Allentown, Pa. 18195 USA.
A preferred source of fully hydrolyzed polyvinyl alcohol resin is 1000 series VINEX(trademark) thermoplastic polyvinyl alcohol copolymer resins manufactured by Air Products and Chemicals, Inc., Polymer Chemicals Division, Allentown, Pa. 18195 USA.
The spacers in accordance with the present invention are preferably fabricated by injection molding. The desired neat polyvinyl alcohol resin or a blend described above is used to provide a molded configuration described hereinbelow. Each spacer is attached to the runner system with a tab or a edge gate.
The spacers are degated from the runner system by initially cooling the surface of the total runner system with parts attached to a temperature at which the gates become brittle. This is accomplished for example, by spraying liquid nitrogen over the surface of the entire runner system, whereby the thin gate sections reach essentially the temperature of the liquid nitrogen entirely therethrough and become very brittle. Tumbling of the runner system while liquid nitrogen is being sprayed thereon or immediately thereafter, causes parts to degate from the remainder of the runner system, breaking away at brittled thin gate sections. Parts can then be easily separated from the remaining runner system by using a wide mesh sieve which traps the runner system at the top thereof. Parts are collected into a container at the bottom of the sieve and placed into an oven at about 170xc2x0 F. for 15-30 minutes and slowly cooled down to room temperature to prevent any condensation of moisture on the just degated spacers using liquid nitrogen technique.
As an alternate degating technique, the spacers can be automatically degated during molding operation by utilizing a sub-gate (tunnel gate).
In either technique the remainder of the runner system after degating the part can be pelletized and recycled into the molding process.
As an alternate embodiment, the spacers as molded can include ridges therein along one or preferably on both opposing surfaces. These ridges perform three functions as follows:
1. When in place around component leads, gases emitted during the soldering process will have an avenue of escape through the recessed area between the ridges. This prevents and minimizes rejects due to solder defects in the finished printed wiring board assembly.
2. The ridges and recessed surfaces provide for increased exposed surface area, which helps prevent premature melting of the spacer by keeping it cool during flow/wave solder process. This prevents or minimizes rejects due to the components dropping down below acceptable limit in the finished printed wiring board assembly.
3. Greater surface area available to the water solvent during rinse and water wash step, results in more rapid dissolving of the spacer material. This helps in faster removal of the spacer from the printed wiring board assembly, and accounts for fabrication process to be cost effective.